Thin substrate image sensors were designed to improve the calorimetric performance characteristics of the sensors by enabling the sensor to be lit by the rear face through a very thin silicon layer; this arrangement provides a way of avoiding the dispersion of the photons and of the electrons photogenerated in the substrate, and therefore avoiding image crosstalk which would greatly impair the colorimetry since the pixels of adjacent images correspond to different colors.
The fabrication of an image sensor on thinned substrate generally comprises the following steps: the process begins with a silicon substrate (solid monocrystalline or silicon on insulator SOI substrate, for example), with a thickness of a few hundreds of micrometers, supporting the industrial manipulation of wafers of ten or twenty centimeters in diameter, this substrate being coated on a front face with an epitaxial layer of monocrystalline silicon which will contain the active circuits of the sensor. In this epitaxial layer, from the front face, the electronic circuitry needed for the various functions of the sensor (image capture, signal processing) is produced. Then, the substrate is glued, by its front face which supports this circuitry, onto a transfer substrate of sufficient thickness for industrial manipulation, and the original silicon substrate is thinned to a thickness of a few micrometers. The very fine thickness of silicon that results from this would not allow for industrial manipulation of the wafer, and this explains the presence of the glued transfer substrate. The very fine thickness, in the case of a color image sensor, serves to considerably enhance the calorimetric qualities of the sensor, the sensor being lit by the rear face, through a layer of colored filters deposited on this rear face and through the very fine layer of epitaxial silicon.
One of the problems encountered in this technology is that of producing contact terminals to provide external electrical connection for the sensor.
One solution already proposed consists in providing contact metallizations in processing steps performed on the front face, and then cutting out, in the thinned silicon, from the rear face, wide and deep openings that expose these metallizations. A solder wire can then be soldered to the metallizations inside these openings. However, this often requires the provision of wide openings (typically 120 to 180 micrometers in width). The total width of the terminals formed in this way is then far greater than the width conventionally provided for connection terminals of conventional CMOS electronic circuits (typically, terminals 60 micrometers wide are sufficient).
Furthermore, the openings must be made before the color filters are deposited onto the sensor; now, the presence of these openings disrupts the uniformity of distribution of the filtering layers; furthermore, the deposition of these filtering layers will leave residues in the openings that cannot easily be removed although it is essential to remove them to ensure the soldering of the connection wires.
Another solution could be envisaged, consisting in forming openings when processing a front face, that is, before the transfer onto a transfer substrate; the openings are made at the position of the connection terminals, through the entire depth which remains after thinning, and the bottom of these openings is metallized. After transfer and thinning, the metallized bottom is accessible on the thinned rear face and constitutes the connection terminal, this time in the same plane as the exit face and not at the bottom of an opening. However, such a method requires steps that are not conventional in a typical CMOS industrial method in that the silicon trenches supporting the sensors could not easily be inserted into an industrial production where both sensors of this type and more conventional circuits (non-thinned substrates) must be produced.